1. Field of the Invention
The present invention relates to semiconductor wafer processing, specifically the cleaning process of a semiconductor wafer.
2. Background Information
In the manufacture of semiconductor devices, the surface of semiconductor wafers must be cleaned of wafer contaminants. If not removed, wafer contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.
One system used to remove wafer contaminants is called a scrubber, wherein both sides of a wafer are scrubbed. In this particular type of scrubber a semiconductor wafer is scrubbed simultaneously on both sides by brushes. Since the wafer is being scrubbed simultaneously on both sides by the brushes there must be a way of holding the wafer in place and rotating the wafer so the entire surface of the wafer is cleaned. A mechanism used for this purpose is called a roller.
The scrubber, wherein both sides of a wafer are scrubbed usually comprises a conveyor type mechanism, rollers, and brushes. In general, the wafer lies flat on the conveyor mechanism and the conveyor mechanism moves the wafer into the brushes. While being scrubbed, the wafer is supported (or held horizontally) by the conveyor mechanism, brushes, rollers, or a combination thereof. As the wafer is being scrubbed by the brushes the roller rotates the wafer so the entire wafer surface may be cleaned. The roller itself is being rotated about its central axis by a stepper motor. The rotary motion of the roller is then transferred to the wafer when the edge of the roller comes into contact with the outer edge of the wafer.
FIG. 1 illustrates a prior art roller. The top and bottom surfaces 110 and 120 of the roller are generally flat and the outer edge 130 of the roller has a slight concave indentation (concave outer edge). As is shown in FIG. 1, the concave outer edge of the roller contacts the outer edge of the wafer. When the roller and wafer contact each other, friction between the edges is created, so that the rotation of the roller causes the wafer to rotate. As can be seen from the Figure, the wafer and roller are in essentially single point contact.
One problem with the prior art roller is that the roller/wafer contact may be insufficient, so that the wafer hesitates, i.e., fails to rotate. This problem is worsened when certain cleaning solutions are used, for example ammonium hydroxide (NH.sub.4 OH). The NH.sub.4 OH or other cleaning solution acts as a grease between the outer edge of the wafer and the outer edge of the roller, reducing the friction between the edges when they contact. Instead, the rotating roller continually slips off the edge of the wafer without imparting any motion to the wafer.
Another problem with the prior art roller is that the roller tends to stall (hesitate) when the roller is in the flat trying to regain the radius. FIG. 2 illustrates this problem. Most semiconductor wafers have a small portion of their outer edge which is flat, e.g. flat 251. When the roller 200 reaches the point P on the flat 251 the wafer stalls (hesitates) until the roller is able to regain the radius. In other words the wafer hesitates until the roller can reach the curved portion of the outer edge of the wafer 250. This problem arises because the roller is downstream of the brush, such that the brush pushes the wafer into the roller. Therefore, when the flat first encounters the roller, it is pushed into it such that the roller can continue to rotate the wafer along the flat (so long as the above described slipping does not occur). However, the prior art roller may not have enough of a grip to push the wafer back toward the brush to regain the radius.
If the wafer fails to rotate during the scrub operation, some areas of the wafer will not be scrubbed, so that the wafer is not cleaned to the level desired. An additional problem which may occur is that if the system relies on the roller to rotate the flat into a predetermined position for the next operation, the flat may be misplaced. For example, after the scrub operation, the wafer may next go to a spin dry operation where it is held by its edges. If the flat is misplaced, the wafer may not be held properly.
What is needed is a method and apparatus for rotating the wafers in a scrubber, wherein both sides of a wafer are scrubbed without slipping or hesitating.